1. Field of the Invention
The present invention relates to an automatic transmission that transmits motive power of a prime mover in a state in which the speed of the motive power is stepwise changed, and more particularly to an automatic transmission including first and second gear mechanisms provided separately from each other.
2. Description of the Related Art
Conventionally, as an automatic transmission of this kind, one disclosed in the Publication of Japanese Patent No. 4875384 is known. This automatic transmission is used for transmitting motive power of an internal combustion engine, which is a motive power source of a vehicle, in a state in which the speed of the motive power is stepwise changed, to a drive shaft, and is comprised of a first gear mechanism, a second gear mechanism, a first clutch, a second clutch, and a shift device. The first gear mechanism includes a first drive gear group formed by a first main shaft connected to the engine, a transmission output shaft that extends in parallel with the first main shaft and is connected to the drive shaft, and a first drive gear group formed by a plurality of gears rotatably provided on the first main shaft, and a first driven gear group formed by a plurality of gears that are fixed to the transmission output shaft and are in mesh with the gears of the first drive gear group, respectively. Gear trains of a first speed position, a third speed position, and a fifth speed position are formed by the first drive gear group and the first driven gear group. The first clutch is a hydraulic multiple-disc clutch which is engaged and disengaged to thereby connect and disconnect between the engine and the first main shaft.
Further, the second gear mechanism is configured similarly to the first gear mechanism, and includes a second main shaft connected to the engine, the transmission output shaft, a second drive gear group formed by a plurality of gears fixed to the second main shaft, and a second driven gear group formed by a plurality of gears that are rotatably provided on the transmission output shaft and are in mesh with the gears of the second drive gear group, respectively. As described above, the transmission output shaft is shared as an output shaft by the first and second gear mechanisms. Further, gear trains of a second speed position and a fourth speed position are formed by the second drive gear group and the second driven gear group. The second clutch is a hydraulic multiple-disc clutch which is engaged and disengaged to thereby connect and disconnect between the engine and the second main shaft. Furthermore, the shift device is capable of executing an operation for connecting one of the gears of the first drive gear group to the first main shaft, and an operation for connecting one of the gears of the second driven gear group to the transmission output shaft.
In the conventional automatic transmission configured as above, when in a neutral state in which neither the first drive gear group nor the second driven gear group is connected, the first and second clutches are disengaged. Further, for the standing start of the vehicle, one of the gears of the first drive gear group for the first speed position is connected to the first main shaft by the shift device, and upon the lapse of a predetermined time period after completion of the connection, the first clutch is engaged.
Further, it is envisaged that in an automatic transmission, such as the above-described conventional automatic transmission, which includes the first and second gear mechanisms and the first and second clutches, during stoppage of the vehicle, there is performed the following control operation: During stoppage of the vehicle, when the shift position of a shift lever is located at a parking position or a neutral position, the first and second clutches are disengaged, whereby the engine is disconnected from the first and second gear mechanisms, and the connection of gears in the first and second gear mechanisms is released (neutral state).
Further, during stoppage of the vehicle, in a case where the shift position has been switched to a drive position, if a request for the standing start of the vehicle has not been made by the driver, to quickly start the vehicle in response to a request made afterward for the standing start of the vehicle, in the second gear mechanism, a gear for the second speed position is connected beforehand to the transmission output shaft as a gear for the standing start of the vehicle, while the second clutch is controlled to a half-clutch state without being completely engaged, and the first clutch is disengaged. In this case, it is envisaged that with a view to quickly transmitting a larger torque to the drive wheels at the standing start of the vehicle performed afterwards, to make it possible to quickly transmit motive power of the engine to the drive wheels at a transmission gear ratio of the first speed position, not only the gear for the second speed position is connected beforehand to the transmission output shaft but also the gear of the first gear mechanism for the first speed position is connected beforehand to the main shaft.
In a case where the control operation as described above (hereinafter referred to as the “during-stoppage control operation”) is performed, for example, when wet multiple-disc clutches to which lubricating oil is supplied are used as the first and second clutches, there arise the following problems: The clutches of this kind are in a slightly engaged state even when they are completely disengaged, since the transmission of the motive power is not completely disconnected due to friction between friction plates of the clutches and lubricating oil (hereinafter referred to as the “clutch friction”). The clutch friction becomes larger as the amount of lubricating oil supplied to the clutches is larger, whereby the motive power transmitted via the clutches becomes larger. Further, in general, the amount of supplied lubricating oil becomes larger as the degree of engagement of the clutches is larger, since friction plates of the clutches have to be cooled by the lubricating oil.
To solve the above-described problems, in the during-stoppage control operation described above, e.g. when the second clutch is controlled to the half-clutch state before completion of connection of the gear for the first speed position, the amount of lubricating oil supplied to the first and second clutches becomes larger according to the control operation, whereby the clutch friction of the two clutches becomes larger, which in turn increases the motive power transmitted from the engine to the first and second main shafts via the first and second clutches. On the other hand, the vehicle is at a stop, whereby the rotational speed of the gear for the first speed position connected to the drive wheels is equal to 0, and hence connection of the gear for the first speed position and the first main shaft is performed in a state in which a relatively large differential rotation occurs therebetween. This causes large vibration even though the vehicle is at a stop, resulting in the degraded marketability. Such problems can similarly occur also when the first clutch is controlled to the half-clutch state before completion of connection of the gear for the second speed position in the during-stoppage control operation.
Further, there is a fear that the problems as described above occur also when dry multiple-disc clutches to which lubricating oil is not supplied or electromagnetic clutches are used as the first and second clutches. More specifically, one of the first and second clutches is sometimes not completely disengaged immediately after stoppage of the vehicle. In such a case, the motive power of the engine is transmitted to the one of the first and second main shafts, to cause the same to rotate. As a consequence, in this case as well, connection of the gear for the first speed position (gear for the second speed position) and the first main shaft (transmission output shaft) is performed in a state in which a relatively large differential rotation occurs therebetween. This can result in generation of large vibration and degraded marketability.